Method for operating an internal combustion engine

ABSTRACT

The present invention relates to a method for operating a multi-cylinder internal combustion engine having gas exchange valves which are variably adjustable with respect to the valve opening characteristics either directly electromag-netically or by means of an electrohydraulic valve actuation system which includes several electromagnetic valves. For adaptation of all gas exchange valves to a desired valve opening characteristics, the electromagnetic valves are operated cylinder-selectively in the operation of the internal combustion engine by means of variable actuating voltages and/or actuating currents.

[0001] The present invention relates to a method for operating aninternal combustion engine according to the preamble of patent claim 1.

[0002] In the journal ‘Auto Motor und Sport’, volume 17, 1999, page 49,an electrohydraulic valve actuation system for an internal combustionengine has already been described which includes a tappet actuated by acamshaft that does not act directly on a gas exchange valve (inletvalve) in the cylinder head of the internal combustion engine but bymeans of hydraulic oil (motor oil). The pressure of the hydraulic fluidpropagates by way of a brake piston to the gas exchange valve, and thisquantity of hydraulic oil and, thus, also the stroke of the gas exchangevalve in the cylinder head can be varied in response to the valve'sswitch position of an electromagnetic valve integrated in the cylinderhead. Variations of the valve opening times cannot be ruled out due tomanufacturing tolerances for the above-mentioned components so thatunequal cylinder fills will automatically lead also to a spreading ofthe exhaust emissions, especially with respect to the multi-cylinderconstruction of the internal combustion engine.

[0003] In view of the above, an object of the present invention is todevelop a method for operating a multi-cylinder internal combustionengine which permits adapting the cylinder fills of all engine cylindersso that the above-mentioned shortcomings are avoided.

[0004] According to the present invention, this object is achieved by amethod with the features of patent claim 1.

[0005] Further features, advantages and possible applications of thepresent invention can be taken hereinbelow from the description of anembodiment explained with reference to several accompanying drawings.

[0006] In the drawings,

[0007]FIG. 1 is a diagrammatic sketch of an electrohydraulic valveactuation system.

[0008]FIG. 2 is a view of the variable valve adjustment of the gasexchange valves resulting from the valve actuation system of FIG. 1,illustrated by several valve stroke curves.

[0009]FIG. 3 is an explanation of individual process steps by way of aprogram flow chart which permit rendering uniform or synchronizing thevalve control times of all engine cylinders in consideration of minimumexhaust gas values.

[0010]FIG. 4 is a current curve, voltage pulse curve and strokecharacteristic curve, representative of the program flow chart of FIG.3, for one of the electromagnetic valves of the valve actuation orinjection system.

[0011]FIG. 1 discloses a diagrammatic sketch of an electrohydraulicvalve actuation system, including a valve drive unit that is arranged inthe cylinder head 1 of an internal combustion engine, comprised of acamshaft 2, a tappet assembly 3, and a gas exchange valve 5 which, inthe capacity of an inlet valve, extends into the intake port 4 of theinternal combustion engine. The gas exchange valve 5 is not actuateddirectly by the tappet assembly 3, but is actuated by means of apressure fluid volume provided by the engine oil pump 6 so as to bevariable with respect to the sequence of motions. To this end, anelectromagnetic valve 7 is inserted into the cylinder head 1 in order tovary the pressure fluid volume compressed between the tappet assembly 3and the gas exchange valve 5. Because the internal combustion engineincludes several cylinders, there is also multiple provision of theinitially mentioned other components of the electrohydraulic valveactuation system corresponding to the number of the gas exchange valves.Besides, the valve actuation system includes per engine cylinder oneintermediate storage 8 that takes up superfluous pressure fluid volume,if any, which is not required for the control of the valve actuationsystem. In addition, an injection valve 20 is mounted in the intake port4 which, exactly as the electromagnetic valve 7, can be operated bymeans of a variable actuating voltage and/or a variable actuatingcurrent to adjust all engine cylinders to uniform rates of injection.

[0012] The diagrammatic sketch of the electrohydraulic valve actuationsystem according to the drawings is consequently rated for amulti-cylinder and, thus, multi-valve internal combustion engine interms of control technology, with the objective of influencing the valvestroke electrohydraulically for each engine cylinder. Only a fewmilliseconds are available to actuate the electromagnetic valves 7 athigh engine speeds.

[0013] A suitable method of operation of the internal combustion engineprevents the system tolerances in the actuation control, the magneticcircuit and the component tolerances within the valve drive unit fromcausing an unacceptable spreading of the valve opening cross-sectionsbecause now the hydraulic control pressure between the tappet assembly 3and the associated gas exchange valve 5, according to the presentinvention, is adjusted individually for each engine cylinder bycontrolling the valve actuating voltage applied to the electromagneticvalve 7 or the valve current so that equal valve strokes for all gasexchange valves 5 result per combustion cycle. Theoretically, this wouldbe technically possible also with the aid of travel sensors in the areaof the gas exchange valves. However, this solution is not feasible dueto cost and structural reasons. Also, care should be taken that theexhaust emission is usually adjusted by means of one single lambda probeper cylinder row.

[0014]FIG. 2 shows exemplarily the valve stroke curves which areprincipally adjustable by the preset variable valve actuation systemaccording to FIG. 1. Starting from a maximum camshaft angle illustratedon the abscissa, the valve stroke curves are also plotted for reducedvalve opening clearances of 40 degrees, 80 degrees, and 120 degreescamshaft angles. Along the ordinate, the valve stroke possible for eachcamshaft angle is plotted which, automatically, exhibits the smallestvalve stroke of roughly 3.8 mm with regard to the smallest camshaftangle of 40 degrees.

[0015] According to the present invention, FIG. 3 shows the individualprocess steps for rendering the valve strokes more uniform and, thus,the valve opening times for all gas exchange valves 5 of amulti-cylinder internal combustion engine which is preferably equippedwith the electrohydraulic valve actuation system known from FIG. 1. Inconsideration of the program run according to FIG. 3, theabove-mentioned system-induced imponderabilities and tolerances in theactuation control of the electromagnetic valves 7 and in the valve driveunit may be adjusted so that each valve actuation system is selectivelytuned to an optimum exhaust emission, with the internal combustionengine running, and the actuation parameters for the electromagneticvalves 7 acquired are stored in a data memory. To this end, the internalcombustion engine is favorably operated in the rotational speed band inwhich inadmissible deviations of the exhaust emission of the individualengine cylinders are the result. The exhaust emission is sensed in a perse known manner by means of a lambda control circuit. The actuatingvoltage or actuating currents of each single electromagnetic valve 7 isthen varied according to the program flow chart and stored in the datamemory in a cylinder-selective manner, and gathered in the way ofparameters as a function of the engine rotational speed. Based on theperformance graph of parameters fixed from cylinder to cylinder, theentire actuation control of the electromagnetic valves 7 takes place.

[0016] The method for determining the exactly synchronized valve controltimes will now be explained in detail by way of the program flow chartaccording to FIG. 3.

[0017] Whenever the internal combustion engine is operated, theelectromagnetic valves 7 of all engine cylinders are initializedaccording to a first operation step 9 specifically for adapting the gasexchange valves 5 with respect to each other. In a second operation step10, the worst exhaust gas value is initiated, and the number of theiteration steps and the iteration step width is determined. In a thirdoperation step according to block 11, it is found out whether the enginespeed is within a predetermined rotational speed band. When thiscondition is not satisfied, a new polling of the engine speed out of theengine control device is made by means of loop 11a. Only if the internalcombustion engine is in the predetermined rotational speed band, whichis especially critical in terms of exhaust gas and where a process ofadjustment of the electromagnetic valves 7 shall be carried out, willthe exit to a subroutine according to operation step 12 follow. In saidstep 12, a currently valid and stabilized exhaust gas value is read intoa data memory of the engine control device, what can e.g. be done by wayof linking to a lambda control circuit of the engine management.Subsequently, it is checked in the following block 13 whether thecurrent exhaust gas value is better than the previously stored exhaustgas value. When this request is satisfied, the current actuation valuefor the electromagnetic valve 7 being activated is stored in the nextstep 14 as a function of the engine speed and the associated enginecylinder. If, however, the request for an improved exhaust gas value isnot satisfied after step 13, the iteration method and, hence, the valveadjustment for the currently concerned engine cylinder is continued byway of loop 14a instead of step 14. It is checked in operation step 16whether all iteration steps have been processed. Unless all iterationsteps have been processed, the valve adjustment process will be repeatedstarting from block 2 by way of loop 16a. If, however, all iterationsteps have been completed, the next electromagnetic valve 7 will bepicked up according to field 17. It is checked in step 18 whether theelectromagnetic valves 7 of all engine cylinders are adapted. In thenegative, the sequence diagram is then repeated commencing operationstep 10 by way of loop 18a. If, however, the adaptation of all enginecylinders is completed, the valve adaptation method explained isterminated with step 19.

[0018] When this valve adaptation process for the individual enginecylinders is gathered by an appropriate algorithm, the offset of thevalve control times in relation to a nominal specification, i.e.,determining only crankshaft angles of rotations, may be determined in acomparatively simple manner in order to adjust the optimizing parameter‘exhaust gas quality’ in this case.

[0019] In an extension of the basic idea, values for differentrotational speed ranges can be determined and stored in a data memory ofthe engine management or engine control device. A performance graph or aset of parameters for a mathematically description may be found outthereby.

[0020] The algorithm can be used in a test run to determine theparameters. In addition, the algorithm may also be used in the normaloperating mode of the internal combustion engine in order to optimizethe parameters, e.g. to counteract the aging of components. To this end,it would be necessary to modify the operation step 2 according to FIG. 3and to indicate the engine speed as an index in the performance graph.

[0021] To sum up, a method for operating an internal combustion engineis shown which permits optimizing the exhaust gas values by variation ofthe actuation times of the electromagnetic valves 7 and, hence, thesynchronous actuation of the gas exchange valves 5 (inlet valves). Thisis done by varying the actuation parameters of the electromagneticvalves 7 in a search operation described in FIG. 3. The result is anoptimal valve actuation control for a quality criterion or also forseveral quality criteria.

[0022] With regard to the program flow chart according to FIG. 3, anoptimized current characteristic curve for each engine cylinder willresult according to FIG. 4 for the electromagnetic valve 7 beingrespectively activated, wherein the optimal current variation isdetermined as a function of time and, hence, proportionally to theengine crank angle as well as by the trigger point T. The result of theadaptation process according to the present invention is a saw-toothcurrent variation characteristic curve which commences with acomparatively low dead current I1 (starting current), which along withthe rise to the exciting current I2 simultaneously causes movement ofthe magnet armature of the electromagnetic valve 7 and keeps it in theopen position until the trigger point T is reached due to the decreaseof the exciting current I2 to the holding current I3 which, in itsamount, is slightly higher than the dead current I1, with the resultthat the magnet armature of the electromagnetic valve 7 moves to resumeits original inactive position. Due to the method illustrated in FIG. 3,the trigger point T is gathered in a data memory of the engine controldevice for each electromagnetic valve 7 and, thus, for each gas exchangevalve 5 in the engine cylinder. The time variation of the current pulseand the movements of the magnet armature are phase-identically plottedbelow the current characteristic curve, whereby a direct allocation ofthe current pulse duration and the magnet armature movement to thecurrent characteristic curve is rendered possible.

[0023] To sum up, a valve actuation method is achieved wherein theexhaust emission is measured for each engine cylinder, and whereinsubsequently—with the objective of reaching optimized exhaust gasvalues—the actuating voltage or the actuating current is alternatinglyvaried as a function of the engine crank angle for each electromagneticvalve 7, and the optimal trigger point T is determined. The optimalswitch points of the electromagnetic valves 7 determined during theprocess are thus acquired individually for each engine cylinder andmemorized as a field of parameters in the data memory of the enginecontrol device as a function of the engine speed. Based on this fixedfield of parameters, a cylinder-selective valve actuation control willthus be effected which, in the present example, finally leads to equalvalve strokes of the gas exchange valves 5.

[0024] It is, however, not absolutely necessary that the valve strokesof the electromagnetic valves 7 are equal. Instead, they may be variedin conformity to any requirement and request with a view to achievingthe objective. According to the above valve control method, thetolerances of the rate of injection can also be adjusted by acylinder-selective actuation of the injection valves 20.

[0025] The present invention is not restricted to the constructiveembodiment of FIG. 1 but also appropriate for alternative valve driveconstructions which, for example, arrange for a direct electromagneticactuation of the gas exchange valves and either include manifoldinjection or direct injection.

[0026] List of Reference Numerals: 1 cylinder head 2 cam shaft 3 tappetassembly 4 intake port 5 gas exchange valve 6 engine oil pump 7electromagnetic valve 8 intermediate storage 9-19 operation steps 20injection valve

1. Method for operating a multi-cylinder internal combustion enginehaving gas exchange valves which are variably adjustable with respect tothe valve opening characteristics either directly electromagnetically orby means of an electrohydraulic valve actuation system which includesseveral electromagnetic valves, characterized in that theelectromagnetic valves (7) for adaptation of all gas exchange valves (5)to a desired valve opening characteristics are operatedcylinder-selectively in the operation of the internal combustion engineby means of variable actuating voltages and/or actuating currents (I1,I2, I3).
 2. Method as claimed in claim 1, characterized in that theadaptation of all electromagnetic valves (7) is carried out independence on the cylinder-selective exhaust emissions of the internalcombustion engine.
 3. Method as claimed in claim 1, characterized inthat the adaptation of the valve opening characteristics of all gasexchange valves (5) is preferably effected in a data memory of an enginecontrol device, namely, by the following process steps: a) initializethe number of the electromagnetic valves (7) and/or injection valves(20) of all engine cylinders, b) initialize the exhaust gas value withthe highest degree of emission and define the number of iteration stepsand the iteration step width, c) check whether the predetermined enginespeed, especially the engine speed at a high exhaust emission, ismaintained where an inadmissible cylinder-selective spreading of theexhaust gas values is to be expected, otherwise step c) shall berepeated, d) read the currently valid and stabilized exhaust gas valueinto the data memory, e) compare the current exhaust gas value with theexhaust gas value that was previously stored in the performance graph ofthe data memory, f) store the current exhaust gas value in the datamemory, in case of need, in dependence on the engine speed, if thecurrent exhaust gas value is better than the original exhaust gas valuestored in the performance graph of the data memory, g) continueiteration steps and adjust electromagnetic valves (7), h) check whetherfurther iteration steps shall follow, i) check whether the actuationparameters of all electromagnetic valves (7) have been gathered in thedata memory, in the negative: select next electromagnetic valve (7) andcontinue operation in step b).